Vectors

ORST is a parallelogram. U is the midpoint of RS and V is the midpoint of ST. Relative to
the origin O, r, s, t, u and v are the position vectors of R, S, T, U and V respectively.
(a) Express s in terms of r and t.
(b) Express v in terms of s and t.
(c) Hence or otherwise show that 4 (u + v) = 3 (r + s + t)

I got (a) alright, but (b) and (c) are perplexing. I know that the answer for (b) is v = (1/2)(s + t), but I'm struggling to figure out how to get there.